1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a blade tip shroud with cooling and sealing.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, a high temperature gas flow is passed through a turbine to convert the energy from the gas flow into mechanical work to drive the compressor and, in the case of an industrial gas turbine (IGT) engine, to drive an electric generator. The turbine is designed to operate under the highest temperature sustainable since the efficiency of the engine is directly proportional to the temperature of the gas flow entering the turbine. Thus, higher turbine inlet temperatures result in higher efficiencies.
One method of allowing for higher turbine inlet temperature is to provide for maximum cooling of the stator vanes and rotor blades in the turbine, especially the first stage airfoils since these are exposed to the highest temperature. Adequate cooling of the turbine parts also increases the life of these parts, which is very important in an IGT because the long service life is a major factor.
Another important design factor is the leakage flow that passes through the turbine such as the gap formed between the rotor blade tips and the outer shroud surface. Reducing the blade tip leakage not only improves the turbine efficiency but also reduces the heat load that results on the blade tip and the shroud from the passing hot gas flow. Hot spots can occur that will damage turbine parts and reduce the efficiency. Damaged turbine parts such as rotor blades and outer shroud segments shorten engine life. High temperature turbine blade tip shroud heat load is a function of the blade tip section leakage flow. A high leakage flow will induce a high heat load onto the blade tip and shroud. Therefore, blade tip shroud cooling and sealing have to be addressed as a single problem.
In the prior art, a grooved turbine tip shroud includes a plurality of grooves within a range of 90 to 130 degrees angle relative to the shroud backing structure which extends into the flow path for the entire axial length of the blade outer air seal (BOAS). The main purpose in the use of a grooved tip shroud in the blade design is to reduce the blade tip leakage and also to provide for rubbing capability for the blade tip. One prior art method of reducing blade tip leakage is shown in U.S. Pat. No. 4,466,772 issued to Okapuu et al on Aug. 21, 1984 and entitled CIRCUMFERENTIALLY GROOVED SHROUD LINER which discloses a stationary shroud surrounding a rotor with radially extending blades in which the shroud includes a plurality of spaced apart lands that define grooves. The shroud grooves in this patent are formed with straight sealing teeth and un-cooled. One major problem with this design is the formation of secondary hot gas flow recirculation within the grooves. Because of the increase in the turbine inlet temperature in the past few years, cooling for the type of blade tip shroud becomes necessary.
Another prior art reference, U.S. Pat. No. 6,155,778 issued to Lee et al on Dec. 5, 2000 and entitled RECESSED TURBINE SHROUD discloses a turbine blade tip shroud having a plurality of recesses 62 disposed in the panel inner surface of the shroud and extending only in part into the panel radially outwardly toward the panel outer surface. These recesses are provided for reducing surface area exposed to the blade tips so that during a blade tip rub with the shroud, reduced rubbing of the blade tips with the shroud occurs for correspondingly decreasing frictional heat in the blade tip. Reduced frictional heat permits the available cooling of both the blade tip and the turbine shroud to reduce the temperature thereof other than it would be with a continuous conventional shroud without the surface interruptions provided by the recesses. Cooling holes also supply cooling air from above the shroud and into the recesses for internal convection cooling as well as providing film cooling of the shroud inner surface. In the Lee et al invention, the cooling holes are straight and therefore the convection cooling capability is low.